In the template you have specified a custom field from the post {{{poi.props.kerkurl}}}. Please specify a data field instead: {{{poi.data.kerkurl}}}.
I think this may be because there was a bug: the button “POI data field” was suggesting “poi.props.myfield” instead of “poi.data.myfield”, so thank you for bringing this to my attention. The bug is fixed in version 2.91.6, which will be available soon.
Thanks, the docs are actually correct – but they’re for mashup custom fields, not the POI data fields. The POI fields are new, and I haven’t had a chance to document them at all yet, but I will – please bear with me 🙂